How good does a TT have to be for a good cartridge

Well My impression is to put a cartridge on a TT that costs half the price of the table.
Or if TT/arm/cart... all generally equal.
This is just a very rough approximation. And spending equal to the TT is done. And spending a tenth is also done.
Not many folks spend a lot more on the cart than on the TT plus arm.

I actually think it's the other way around. With a cartridge that is just "good", low- to middle-priced, plus a really good tonearm well matched to the cartridge and and an excellent tt, you can get better results than using a very expensive cartridge with mediocre tonearm/tt. So, first get the best tt and tonearm you can afford. Then play with cartridges to your heart's delight (and your ears too).

I hope I accurately figured out what you wanted to know and responded to it.

A better table will improve the sound of a modest cartridge more than a poor turntable will be upgraded by an expensive cartridge.